Changes in the fluorescence of the electrofluorochrome CC6 (3,3'- dihexyl 2,2 oxacarbocyanine) in the presence of rabbit skeletal muscle myofibrils have been shown to parallel the changes in the Donnan potential of glycerinated muscle on changing the ionic strength of the bath solution. The changes in fluoresence seem to be a property of the myosin molecule and occur independently of the presence of actin. A method has been developed for determining the dose-response relationship for both activation and desensitivation of the change in membrane conductance of the frog sartorius muscle in the presence of acetylcholine. The data indicate that activation is satisfied by the equation A (C, ss)/(A (max, ss)-A (C,ss)) equals Ky2 Cn; for normal muscles, n equals 1.2 plus or minus 2 and for denervated muscles n equals 1.0.